As you all may know the DVD specs say
PIF < 4 (1 ECC)
PI < 280 (8 ECC)
for scanning speed of 1x.
When the first LiteOn DVD burners (first drives with error report capabilities) came out, some CD Freaks here said, that 4x scanning is the way to go, because of the amount of time a scan @1x needs. The errors should not be greater than in the specs although scanning with 4x, so that one can be sure to have a very good disc.
This is a definition for some good reasons, but you also can define your own standard and compare all your discs @8x scanning speed. This would mean to scan all your discs @8x with the same drive and same firmware to get comparable results.
To go with the specs you'd have to scan @1x (you play Video-DVDs @1x, so it can't be wrong to check those out @1x, too)...